lib: add deprecated option warning helpers

tests/default.nix

@@ -177,6 +177,7 @@ import nmtSrc {
       (
         [
           # keep-sorted start case=no numeric=yes
+          ./lib/deprecations
           ./lib/generators
           ./lib/types
           ./modules/files

tests/lib/deprecations/default.nix

@@ -0,0 +1,3 @@
+{
+  lib-deprecations-warnings = ./warnings.nix;
+}

tests/lib/deprecations/warnings.nix

@@ -0,0 +1,42 @@
+{ lib, pkgs, ... }:
+
+{
+  nmt.script =
+    let
+      expected = pkgs.writeText "deprecation-warnings.expected" ''
+        Using `programs.example.settings` as a list is deprecated and will be
+        removed in a future release. Please use `programs.example.settings.items` instead.
+
+        Move list entries under `settings.items`.
+
+        The value "kde6" for `qt.platformTheme.name` is deprecated and will be
+        removed in a future release. Please use "kde" instead.
+      '';
+
+      actual = pkgs.writeText "deprecation-warnings.actual" (
+        lib.hm.deprecations.mkDeprecatedOptionValueWarning {
+          option = [
+            "programs"
+            "example"
+            "settings"
+          ];
+          old = "a list";
+          replacement = "`programs.example.settings.items`";
+          details = "Move list entries under `settings.items`.";
+        }
+        + "\n"
+        + lib.hm.deprecations.mkDeprecatedOptionValueRenameWarning {
+          option = [
+            "qt"
+            "platformTheme"
+            "name"
+          ];
+          old = ''"kde6"'';
+          replacement = ''"kde"'';
+        }
+      );
+    in
+    ''
+      assertFileContent ${actual} ${expected}
+    '';
+}